Solid state circuits used to control stepper motors switch the direction and intensity of electrically created magnetic fields to step the shaft of the motor into precise positions.
A typical arrangement 100 for controlling a stepper motor 110 is shown in FIG. 1. A motor controller 101 (e.g. A3979 Microstepping Driver from Allegro Microsystems, Inc.) is provided with +24V driver voltage and +5V logic voltage, and receives control signals 105. Controller 101 advantageously includes a translator 102 for managing internal components (e.g. digital-to-analog converters, not shown) so that a pulse delivered to a STEP input causes the motor to be advanced by one increment. As shown in FIG. 1, four outputs of controller 101 are connected to terminals 111-114 respectively at the coils 103, 104 of motor 110. Controller 101 also provides a controller output signal at HOME terminal 108 indicating a home position for the stepper motor. SENSE terminals 106, 107 are connected to sensing circuits indicating current in coils 103, 104 respectively.
In order to increase the speed and torque of motor 110, the current used to create these magnetic fields exceeds the capacity of windings 103, 104 for handling that current. A conventional solid state controller circuit limits this current to safe levels by rapidly switching the current off and on to limit the average current to both safe levels for the motor and appropriate levels based on the desired position of the motor. This rapid switching creates RF noise that can interfere with the operation of other equipment in the vicinity of the stepper motor and its wiring. This is of particular concern when the stepper motor is used to adjust an antenna system.